Hollow fiber ultrafiltration (UF) or microfiltration (MF) membrane bioreactor (MBR) processes have been used in water and wastewater treatment to provide high levels of finished water treatment. These activated-sludge-type processes typically involve a single basin at ambient pressure containing a series of coarse bubble aeration devices into which single or multiple modules (or groupings) of hollow fiber or plate-type UF or MF membranes are inserted. Waste enters one end of the basin, is mixed with a biomass containing active aerobic organisms, and air is added to provide oxygen. The mixture of biomass and water is referred to as “mixed liquor.” The solids in the mixed liquor are referred to as “mixed liquor suspended solids” (MLSS). During aeration, the membrane devices filter the particles of biomass from the liquid substrate.
These membrane filters are typically composed of polymeric materials formed into hollow structures with pores of 0.01 to 0.4 microns in dimension and are open at one or both ends. Multiple membranes are typically arranged on a series of top and/or bottom manifolds which act to distribute air and water. The membranes are attached to these manifolds through a potting system that is known in the art.
An exemplary MBR of the prior art is shown in FIG. 5 and described in more detail in U.S. Pat. No. 6,245,239, incorporated by reference. Reactor 110 comprises a tank 112 that is supplied with feed liquor 114 through inlet 116. The feed liquor typically comprises solids, which may include microorganisms, suspended solids, or other matter. The feed liquor 114 mixes with the tank liquor 118, which typically has greater concentrations of solids, particularly when used for treating wastewater. One or more membrane modules 120 are mounted in the tank and have one or more headers 122 in fluid communication with a permeate side of one or more membranes 106.
Tank 112 is typically kept filled with tank liquor 118 above the level of the membranes 106 in the membrane modules 120 during filtration. Transmembrane pressure (TMP) (differential pressure across the membrane), created by the suction of pump 130 on permeate line 128, causes filtered water, referred to as permeate 124, to flow through the walls of membranes 106 into headers 122 and out through permeate outlet 126. Solids are retained on the surface of the membrane. Periodically, the membranes are cleaned by a reverse flow of permeate 124 or by air sent back through the membrane and out through the filtering pores to remove accumulated matter.
In many systems, such as the one shown in FIG. 5, air from an aeration system 137, comprising an air source such as a blower 142, is bubbled through aerators 138 installed on an aerator manifold 151 located below the membrane modules. Bubbles 136 created by the aerator scour the outer surface of the membranes to remove accumulated particles while also providing oxygen transfer to meet the biological oxygen demand of the tank liquor. An air-lift effect caused by the decreased local density of the water induced by the air bubbles also creates a recirculation pattern 146 in the tank. Typically the hydraulic design of the basin is optimized for the flow velocities required to maximize either the air scouring operation or to keep a specific velocity across the membranes. In some cases air is used in constant or cyclic action to induce the velocities necessary to optimize the filtration process. In most cases coarse bubble diffusers or nozzles located on the basin floor directly below the membrane assemblies are employed to provide a large bubble with features that optimize scouring of the membrane and provide the velocity to induce updraft effects or cross flow action, while still providing oxygen for aeration. Occasionally, a portion of the mixed liquor will be withdrawn at predetermined levels through drain 134 as controlled by drain valve 132 to maintain a specific level of MLSS, typically in the range of 10,000 to 20,000 mg/l.
The designs of such bioreactor basins are typically optimized for the air flow patterns and velocities required by the membranes to achieve required performance. Because these systems are designed to use air to optimize membrane performance, aeration for biological needs is typically a secondary consideration. Because in some cases coarse bubble or nozzle diffusers are used to provide the oxygen, these systems do not optimize the efficiency of the aeration mechanism, resulting in a biological process that is inefficient. Typically a major portion of the costs of operating these systems is the cost of providing air for the biological process through aeration, air for the membranes for air scour and/or backwash, and/or water for backwash.
Although it is known that less aeration is required for scouring the membranes when the solids load is minimal (see, e.g., U.S. Pat. Nos. 6,303,035 and 6,375,848), applications with a high MLSS typically utilize substantial coarse bubble aeration, such as is described, for example, in U.S. Pat. No. 6,193,890 and others.
One process, known as the AquaMB Process™ promoted by Aqua Aerobic Systems, Inc., comprises a first stage bioreactor operated to have alternating aerobic and anoxic periods. Following quiescent settling, supernatant from the bioreactor is transferred to a 10 micron cloth media filter. Filtrate from the cloth media filter feeds a membrane system. Although the cloth media filtration step decreases the solids load on the membrane filters (thus allowing fewer membranes and requiring less frequent cleaning of the membranes), the cloth media filtration step adds significant capital and operating expense to the process.
In another biological reactor system design described in U.S. Pat. No. 3,472,765, a stream of high-solids material may be withdrawn from a pressurized biological reactor vessel and recirculated through a membrane separator loop under greater than atmospheric pressure, where turbulent flow through a membrane separator inhibits build-up on the surface of the membrane. The pressure vessels, pumping, and piping required for both the biological portion and the membrane filtration portion of this process are capital and energy intensive for large volume effluent treatment.
Another method for treating wastewater effluent is known as a Sequencing Batch Reactor (SBR). An exemplary SBR is described in U.S. Pat. No. 4,468,327, which discloses an activated sludge wastewater treatment process for reducing pollutants in municipal and industrial waste. The effluent treatment system disclosed in the '327 patent, shown schematically in FIGS. 1 and 2, comprises a tank 1 having a length 3.5 to 6 times its width, an inlet 2 at one end 3, and a decanter 12 at the other end 13. The tank comprises a plurality of submerged air diffusers 7, supplied by air headers 8 and 9, that typically operate cyclically in a sequence that includes a diffusion or aeration period, followed by a rest or quiescent period, followed by a decant period. Aeration allows the activated sludge to treat the incoming biological waste, settling allows the solids to settle, and decanting drains the upper clarified supernatant from the tank of treated water. The total sequence of aeration, settling and decanting stages may typically take 4 hours, 6 hours or 12 hours.
Systems are sized based on the biological mass necessary to reduce the pollutants to be treated stipulated as the food to microbe (F/M) ratio. The SBR described in the '327 patent typically operates with overall F/M ratios of up to 0.4, with immediate F/M ratios of up to 5.
One advantage of an SBR is its ability to continuously accept inflows of wastewater into a single compartmented tank without requiring upstream load balancing. The length and width ratio of the tank prevents short circuiting. The tank typically includes at least one transverse baffle 4 that divides the tank into a first compartment 33 serving as the inlet and at least a second compartment 6 remote from the first. The second compartment may have additional partial or full transverse baffles (30 and 31, respectively) that divide the second compartment into chambers 34 and 35 to help deal with shock organic loads. The volume ratio of the first compartment to the second compartment is typically between about 1:10 to about 1:3. The first portion is typically dimensioned to operate at high immediate F/M ratios of up to 5 units, with biological activity (measured as oxygen uptake rate in units of milligrams of oxygen per gram of mixed liquor suspended solids per hour) of up to 300 and a solids content of up to 15 pounds of biological solids per square foot of vessel floor area. The tank as a whole typically operates at an F/M ratio of up to 0.4.
U.S. Pat. No. 6,613,222 discloses a process comprising a sequencing batch reactor that omits the decanting stage in favor of a process that has a mix fill step, a react fill step, and a react discharge step. Influent enters the SBR during the mix fill and react fill steps, but not during the react discharge step. Aeration is used during the react fill and react discharge step as desired to create aerobic or anoxic conditions. During the react discharge step, flow is discharged from the reactor and circulated through a membrane filtration unit. In one embodiment, two SBRs are used side by side, with one side operating in the mix fill and react fill steps while the other side operates in the discharge step, and vice versa. Like the system described in the '765 patent, the process disclosed in the '222 patent also uses a membrane system in which the mixed solids is pumped through the filtration unit under pressure to force the liquid through the membrane.
It is desirable to provide a system that utilizes membrane filtration in a way that minimizes capital investment and operating cost by enabling the use of membranes to facilitate effluent treatment in systems with submerged membrane modules at ambient pressure without the need for constant aeration to prevent clogging.